Evaluation of production from a subsurface reservoir utilizes four-dimensional (4D) processing of two seismic datasets obtained at two different times, e.g., two vintages, from a given subsurface region to determine changes in Earth properties resulting, for example, from petroleum reservoir production. The two seismic datasets can be obtained from land-based seismic surveys and marine-based seismic surveys. As the seismic surveys are conducted at different times, variations in the geometries of the two surveys exists, which complicates the comparison of the two seismic datasets. These variations in geometry occur in particular in marine-based seismic surveys.
Marine seismic data acquisition and processing generate an image of a geophysical structure, i.e., subsurface, under the seafloor. While this image does not provide a precise location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of oil and/or gas reservoirs. Thus, providing a high-resolution image of the subsurface is an ongoing process for the exploration of natural resources, including, among others, oil and/or gas.
During a seismic gathering process, as shown in FIG. 1, a vessel 10 tows an array of seismic receivers 11 located on streamers 12. The streamers may be disposed horizontally, i.e., lying at a constant depth relative to the ocean surface 14, or may have spatial arrangements other than horizontal, e.g., variable-depth arrangement. The vessel 10 also tows a seismic source array 16 configured to generate a seismic wave 18. The seismic wave 18 propagates downward, toward the seafloor 20, and penetrates the seafloor until, eventually, a reflecting structure 22 (reflector) reflects the seismic wave. The reflected seismic wave 24 propagates upward until it is detected by receiver 11 on streamer 12. Based on this data, an image of the subsurface is generated.
Alternatively, ocean bottom cables (OBC) or ocean bottom nodes (OBN) and ocean bottom seismometers (OBS) may be used to record the seismic data. FIG. 2 shows an OBC 30 that includes plural receivers 32 distributed on the ocean bottom 20, which may be connected to each other (or may be independent OBN/OBS) with a cable 33 that may also be connected to a data collection unit 34. Various means (e.g., underwater vehicle) may be used to retrieve the seismic data from the data collection unit 34 and bring it on the vessel 10 for processing.
When these marine-based survey techniques are used to monitor a producing reservoir the location of the streamers or nodes may vary between vintages. Variability in the two seismic datasets is also introduced when a first dataset is collected using one technique and the second dataset is collected using the other technique. As 4D processing determines changes in Earth properties by evaluating differences in seismic data acquired at different times but processed together, the success of 4D processing depends on how well differences in acquisition methods and geometries are handled during data processing and imaging.
If these differences are accurately compensated, changes in the subsurface that are related to fluid production can be identified by areas of significant difference between baseline and monitor images after migration. Failure to compensate for acquisition differences leads to the creation of 4D noise, which is an appreciable difference of baseline and monitor migrated images not attributable to reservoir production. Differences in both information content and wavefield sampling lead to this generation of 4D noise. Therefore, it is desirable to address acquisition differences through more accurate methods of data processing.
Accurate selection of subsets of seismic traces in the base and monitor seismic surveys reduces the level of 4D noise in the migrated images by choosing subsets of data that migrate to give the same image, free of the effects of acquisition differences and other sources of 4D noise, and giving a 4D image with a faithful representation of petroleum production activity. Conventional methods utilize 4D-binning to select traces from the base and monitor surveys for further processing based on a set of criteria that assess their degree of similarity. These conventional methods evaluate a set of similarity criteria in the data domain, i.e., before migration, and work well when the base and monitor surveys have similar acquisition geometry, for example, a towed-streamer base survey and a towed-streamer monitor survey acquired in similar positions but at different times. However, when the base and monitor surveys have different acquisition geometries, for example, a towed-streamer base survey and a sparse OBN monitor survey, the surface or data domain trace attributes used to measure similarity in the 4D-binning process are not a good proxy for similarity of the traces in the seismic datasets or for the wavefield sampling in the seismic datasets. Furthermore, the evaluation of similarity using surface or data domain trace attributes cannot accurately measure similarity of information content, since the grouping of traces by surface attributes does not allow the comparison of similar parts of the seismic wavefield.
Therefore, the need exists for improved methods and systems for improving the similarity between two seismic datasets even given changes in acquisition geometries, e.g., towed-streamer and ocean-bottom data. These improved systems and methods would be applicable to 4D processing for seismic datasets associated with multiple seismic survey vintages conducted at different times.